Exhaust muffler



EXHAUST MUFFLER Y 29 EW. Schnell K BYE ATTORN EYS Patented June 23, 1931 UNITED STATES PAT-ENT OFFICE FREDERICK H. SCHNELL, OF MADISON, WISCONSIN, ASSIGNOR TO F. BURGESS LABORATORIIB, INC., OF MADISON, WISCONSIN, A CORPORATION F DELAWARE Application med May 8, 1929. Serial No. 361,376.

M invention relates to muiers for noisepro ucing gases and more particularly to exhaust muiilers for expanding and noiseproducing pulsating gases such as are discharged from internal combustion engines.

My improved exhaust muffler departs very decidedly in construction from the exhaust mufllers heretofore employed.

lThe explosion within the cylinder of an internal combustion engine creates tremendou'spressure therein. The steam or vapor in thecylinder of a steam 'engine isalso under' very high pressure. Upon the opening of the exhaust port, the gas emerges at high velocity, very quickly reducing the pressure in the cylinder. By the word' gas, Imean to embrace both gas and vapor. The result is a succession of pulses of gas at high pressure traveling through the exhaust manifold or pipe or any other type of exhaust system at a high velocityg Upon the emergence of the gas pulse into the atmosphere an addii tional spurt in velocity is probably acquired by reasonof expansion, and a sharp sound results. The greater thev pressure gradient between the pulse of gas and the atmosphere the greater the velocity and the sharper the sound. In order to successfully silence exhaust noises the'muiiler must suppress the pressure peaks and thus even the flow of the escaping gas pulses to the point lwhere a sound. v is not created. In addition, I have found that sound communicated to or generated within the 'muiller should be silenced.

5 the desired result by an or all of the following methods: cooling t e exhaust gases and diminishing "their volume; allowing lthe gases to expand and reducing their pressure while confined; creating eddy currents and internal friction within the gases;l causing friction between the gases and .the walls and passages within the muiier; and/or `im-v peding the forward fprogress of the gases by the interposition o baies in their path.

The typical muler of today is a metal shell connected in the exhaust system of an engine' which'may incorporate one or more ofthe followingffeatures: an expansion and mixing chamber followed by asmall outlet ya The muiilers of the present day accomplishA therein. The types embodying tortuouspaths, obstacles, and baiies, retard the flow of gas, and in so doing, build up a back pressure Within the muflier and reduce the efficiency of the engine. There is a muler being produced which actually assists in the scavengingL of' the cylinder but the scope ofr its application is limited to special types vof engines of known, constant speed.

It is an object of my invention to provide an exhaust muffler whose scope of application extends to practically all ofthe common types of explosionl and steam engines, to fire arms, and to air intakes and exhausts as in air compressors.

Itis a further object of my invention to provide a muiiier which will not build up suicient back pressure to appreciably reduce the efliciency of the engine.V

It is a further object of myinvention to provide a muiiier which will silence explosion noises lmore effectively than do the mufiers at present employed.

It is a still further object of my invention to provide a 'muiler which is more simple in construction than are the present mufv Fig. 3 is a longitudinal, sectional view lof vanother form of the invention;

Fig- 4 is a longitudinal, sectional View of.'`

another form of my invention; A

Fig 5 iS an end view of Fig. 4.-; Y l. l

' Fig. 6 is a longitudinal, sect-ional View of another form of my invention;

Fi 7 is a transverse, sectional view on line -7 of Fig. 6;

Fig. 8 is a longitudinal, sectional View of another form of my invention;

Fig. 9 is a longitudinal, sectional View of another form of my invention; l

Fig. 10 is a longitudinal, sectional view of another form of my invention;

Fig. 11 is a transverse, sectional View on lines 11-11 of Fig. 10;

Figs. 12 and 13 are charts which illustrate thel effect of my muiling device upon exhaust gases.

In the form ofmufHer for internal combustion engines shown in Figs. 1 and 2 of the drawings, an outer sheet metal cylinder l, which is Welded or otherwise fastened at its ends to annular metal discs 5 and 6, encloses an intermediate annular spacev 2 packed with a porous gas-pressure absorbing and soundabsorbing material which may be a mineral fibre, steel, copper or other metallic Wool or mixtures thereof, or any other porous gas-pressure absorbing and sound absorbing, non-flammable material such as sized crushed mineral matter, mica, exfoliated vermiculite such as zonolite, blown slag, coke, pumice or other porous aggreates and built-up units of such aggregates.

or silencing blow-off gases, air compressor intake and exhaust noises, and noises produced by gases at room temperature, flammable sound-absorbing materials such as Wotl, cotton, or other cellulosic fibers may be use In view of the fact that every material which is not a perfect reflector of sound is, to a degree, an absorber of sound, it would probably be well to draw some sort of a specification for the term sound-absorbing material as that term is used in the resent case. In all acoustic data the sound transmitted by an open Window is used as the standard of comparison. The ratio of the sound absorbed by an area. of a material to that transmitted by an equal area of open window is called the absorption factor or value of that material. If a material one square foot in area absorbs one-fourth the sound transmitted by one square foot of open window, that material is said to have an absorption factor or value of 25 per cent. 'In this specification Whenever the term sound-absorption factor or value is used in conjunction with an absorbing material or construction in amuiler, it is to be understood that this factor or value is obtained by measuring the absorption of such material or construction in the usual Way by means ofV flat pads and the like of identical material and construction. This is necessary since it is very diiiicultvor impossible to obtain the factor after the muffler is assembled. In the muiiers of my invention as the Sound absorption factor of the sound absorbing material decreases, the size of the muier must increase and a material With a factor of 10 percent or less would probably necessitate Ian unwieldly size. Hence, in the present consideration, by sound-absorbing material I do not contemplate a material having an absorption factor of less than 10 percent at 1024 double vibrations per second. Throughout this specification all absorption values are based on 1024 double vibrations per second. For most situations an excellent muHer may be constructed with a material having an absorption factor of 25 percent or more. For exceptional results I prefer to use a material having a factor greater than 45 percent. The absorbing material should be of appreciable thickness for best results. It should be at least onequa-rter of an inch thick and preferably onehalf inch or more. By mineral fibre I contemplate natural or artificial mineral Wool, shredded asbestos, or any other mineral material of the same general nature.

An inner cylinder 3 of foraminous rigid material such as perforated sheet metal provides a straight, open, unobstructed central duct 4 extending from end to end of the muffler. Because of the high temperature of internal combustion engine exhaust gases, cylinder 3 expands and therefore it preferably is not fastened to either metal disc 5 or 6 or both, thereby forming a slip joint and preventing buckling, since cylinder 1 Which is also fastened to discs 5 and 6 does not expand. Disc 5 may be provided with a threaded central opening to receive pipe 7 which conducts the exhaust gases into the mufller. It is usual in automobile mufllers to provide a slip joint so that pipe 7 is free to move in disc 5 and such construction may be .employed in vplace of the threaded connection shown. The arrows in the drawings illustrate the direction of gas flow. The gases are finally exhausted through pipe 9 in opening 8 of disc 6. It is preferred to have the internal diameter of cylinder 3 of the same size as exhaust pipe 7. If it is smaller it increases the back pressure. As Will be discussed further hereinafter, if cylinder 3 is larger than pipe 7 the acoustic absorption decreases.

As hereinbefore explained, a series of gas pressure peaks emerges at high velocity from pipe 7. Each peak tends to expand in all directions. The gas pressure Waves freely pass through perforated cylinder 3 and enter annular space 2.4 The property of the per forated sheet metal of permitting the free transmission of gas and sound therethrough, together with its other advantages, makes it the preferred foraminous materlal for a device of this character. The perforations may vary in size and shape. Because of 'lower manufacturing costs I prefer circular openings. Holes suitable may vary from approxlmately 0.070 to.t 0.125 inches in diameter but I do not wish to be limited to these dimensions. For most purposes the holes are evenly distributed throughout the area of the sheet metal although such even distribution is not necessary. The holes are present in sufficient number so that their to'tal arearrepresents from 21/2 percent up to 35 percent of the area of the sheet metal though fair results may be obtained with holes having an area as low as 1 percent of the area of the sheet. The shape, size and distribution of the perforations all have an influence on the ease with which gases and sound pass through the sheet of metal. The. holes should be small enough so that the gas-pressure absorbin and sound absorbing material will not wor its way out through them when the mufller is in service. Excellent results are obtained when the ratio of the unperforated portion of the metal forming such facing to the openings thereinis such that a substantiallycontinuous surface is exposed to the sound waves and gas pressure waves. mensions of the individual openings are usually less than the distance between the edges thereof. Tests show that a muffler containing absorbing material faced with a stiff erforated sheet, the perforated area of w ich is as low as 21/2 per cent of the area of the sheet with holes about .075 inch in diameter, absorbs as much sound as and has a muliling efficiency equal to a similar muler in which the perforated facin is omitted. It is possible to omit the per oratedmetal facing or other foraminous facing by constructing a molded annular or other suitably shaped absorbing material as by bondingI sized crushed mineral particles together at their points of contact to form a porous mass. Such a molded gas-pressure absorbing and sound absorbing material has an opening therethrough corresponding to the foraminous tube 3. In another construction metallic fibers, like steelwool, are formed into a mat-- tress by stitching the libres together with a metallic thread such as steel wire. This mattress is used to line the inside of the mufller shell 1. The foraminous duct may be omitted when such a mattress is used.

The fibrous gas-pressure absorbing and sound absorbing material comprises nonflammable fibres distributed in heterogeneous arrangement to form tiny interstices, pores or cells of more orless uniform size and distribution. It is acked in such a manner that usually only rom 1 to 20 percent of annular space 2 is occupied b percent is free space. The packed fibers, dpending on their physicalA pro erties, weigh after packing in the muliler, om 4 to 100 In suchcases the average dithe actual n of the fibres of the material and there ore 80 to 99' pounds per cubic foot. The packed mineral wool weighs from 9 to 36 pounds per cubic foot, the fibers occupying from 5 to 20 percent of the space'. Steel wool weighs from 4 to 100 pounds per cubic foot, the fibres occupying from l to 2O percent of the space. Especially good results are obtained with Steel Wool packed to occupy 2 to 5 percent of the s ace. If the fibres are packed too loosely t ey jar down into a more compact mass after the muliler is put into use, and if they are packed too tightly the gas-presf sure and acoustic absorption are cut down,

thereb l decreasing the muler eiiciency..

The a sorbing material is subjected to violent pounding and vibration by the oscillating influence of the exhaust gases and must not-disintegrate readily under those conditions. Certain types of mineral wool resist this disintegrating action much better than others. However, a metallic wool like steel wool is highly resistant to this action. The sound-absorbing portion of the muier, therefore provides an expansion space for gaspressure eaks and lthe maximum pressure of the pu se of gas is decreased as a result of the expansion, the pressure wave being longer and lower in intensity. Fig. 12 shows in solid lines a typothetical diagrammatic representation of several gas pulses as they enter the muffler and in dotted lines is a similar representation, of the same pulses after expansion has taken place. The pressure wave characteristic has become flattened. I also believe that a second phenomenon takes place Within annular space 2 to further flatten, the wave characteristic. Relatively speaking, the rapidly moving as enters the annular space 2 in the form o a succession of rapid compressions and rarefactions. The compressions are suppressed and merged'together by the choking effect of the walls of the tiny pores or cells 'with a resulting re-` duction of theAv intervening rareactions. Fig. 13 shows in solid .lines the pressure wave characteristic corresponding to the dotted curve of Fig. 12. The dotted lines of Fig. 13 show the pressure wave characteristic after the gas has undergone the choking action of the porous 'gas-pressure absorbing and sound-absorbing material. The porous material offers the fairly stea ow ofl the gas stream throu h the straight and unobstructed duct 4, andg especially if duct 4 is made of smoothl perforated, sheet metal, it facilitates the flow,-

with the net result that there is practically no resistance to the gas flow and little or no back pressure is built up aside from that due to the surface friction. The absorption as-pressure peaks bythe absorbing materia to smooth out the gas flow and elim- (practically no resistance to r inate the noise made by the gas slugs when emerging into the atmosphere is-an important feature of my invention. I use thev sound-absorbing material for this double function without appreciabl increasing the back pressure. I believe thls to be entirely new in the muffler art. Y

I believe that some noise is generated at the end of pipe 7 as explained heretofore in connection with expansion chambers. This noise is silenced by the sound-absorbing material. It is not projected longitudinally of the duct 4 since sound does not project itself in single directions to form rays. It emanates equally in all directions unless it encounters refiecting surfaces. Foraminous cylinder 3 allows this sound to pass freely therethrough and it is thereafter absorbed by the material in annular space 2.

Since the depth to which the gases will penetrate the material in space 2 depends upon the pressure, greater penetration should occur at the end adjacent the pipe 7 For this reason my muffler may take the form shown in Fig. 3 wherein annular space 2 is greater at the end where the gases enter.

yAnother form of muii'ler which I have found to be very effective is illustrated in Figs. 4 and 5. Metallic casing 10, which may be of one piece, but is preferably of two pieces threaded togetherat the middle,

has a threaded opening on its side adjacent one end to receive tangentially the exhaust pipe 11. A portion 12 of the interior of the casing' adjacent the entrance opening does not contain absorbing material. IVithin casing 10 annular disc 13 is fastened. Perforated or foraminous sheet metal tube 14 may be fastened vat its ends to disc 13 and casing 10 respectively. Tube 14 encloses annular space l5 within which the gaspressure 'absorbing and sound absorbing ma- Vterial iss loosely packed. Pipe 16 may be threaded or otherwise fastened in the exit end of casing 10 or it may form a sliding fit therein.

The exhaust gases may enter by way of tangential pipe 11 into expansion chamber 12 and thence continue through tube 14 and pipe 16. Expansion of the exhaust gases takes place in# chamber 12 to effect a considerable reduction in the magnitude and sharpness of the pressure peaks. There undoubtedly is an appreciable amount of noise generated in chamber 12 due to the gas expansion which noise is silenced by the gaspressure absorbing and sound-absorbing material in annular space 15. 'Ihe pressure peaks, as hereinbefore explained, are further reduced by expansion into the gas-pressure absorbing and sound-absorbing material.

A further modification of my invention is shown in Figs. 6 and 7. Outer cylindrical shell 17 is connected, as by welding, to discs 18 and 19. These discs may in turn be threaded to receive outlet and inlet pipes 20 and 21 respectively. Inner perforated shell 22 may be made of ribbed sections with the sections crimped together at the ribs as shown in Fig. 7, the ribs resting upon the inner surface of outer shell 17 and holding shell 22 in place. Within shell 22 the noninflammable gas-pressure absorbing and sound absorbing material is packed as hereinbefore described. The gases pass unobstructed through the annular ductl or channel. The central core absorbing construction of Fig. 6 may be used in place of the annular absorbing construction of Fig. 4 in conjunction with the tangential entrance expansion space with excellent results.

Fig. 8 represents a further modification of my invention embodying the open expansion chamber idea. Outer cylindrical shell 23 is connected, as by welding, to end closure discs 24 and 25 which may be threaded to receive exit and entrance pipes 26 and 27. lVithin shell 23 and coaxial therewith is cone-shaped shell 28 of perforated metal or other foraminous facing with its large end adjacent the gas entrance end of the muiiier. The annular space 29 between the two shells is filled with gas-pressure absorbing and sound absorbing material. The entrant gases are free to expand into a large open space and increasing quantities of absorbing material.,a1'e encountered in which further expansion occurs as the gases proceed along their path to the exit opening. Seemingly inconsistent as this construction is with that shown in Fig. 3, the same principle underlies both. A larger space is afforded the entering gases in the muier shown in Fig. 3 by enlarging the muilier at that end, whereas more space is provided in the device illustrated in Fig. 8 by maintaining the muiier at the same size throughout its length but removing space-consuming absorbing material from ,the gas entrance end.

Fig. 9 illustrates a further modification of my invention embodying a combination of the features illustrated in Figs. 1 and 6. Guter metallic cylindrical shell 30 has opening 31 at one end and'is connected, at its other end to end closure disc 32 which may have a threaded opening to receive inlet pipe 33.- Within outer cylinder 30 and spaced therefromy is inner coaxial cylinder 34 of perforated sheet metal. The annular expansion space 35 is filled with loosely packed gas-pressure absorbing and sound absorbing material. Centrally located Within inner cylinder 34 is cylindrical core 36 of perforated metal which may have a conical end as shown and which is also filled with absorbin material 37. The gas enters and is allowe to expand in space 38 and thereafter follows annular channel 39, lined on both sides with absorbing material, to the exit opening 3l which may open directly to the atmosphere,. or communicate with an exhaust pipe 37. This muiiier shows a high mufliing eiiciency for the spgce occupied by it and 1s well adapted to yused where a high eiciency muiller occupying a small space is needed as onoutboard motors.

Figs. 10 and 11 illustrate a still further modification of my invention. Outer sheet metal shell is Welded or riveted or other- Wise fastened to metallic end pieces 41 and 42 which may be rectangular and which are threaded to receive outlet and inlet pipes 43 and 44 respectively. lVithin shell 40 there is a plurality of parallel spaced perforated metal shells 45 which are filled with loosely packed absorbing material. This muiler also shows a high efficiency for the space occupied by it and is well adapted for use with outboard and similar motors.

Perforated metal sheets 46 may be spaced about the thickness of one of the shells 45 from the` sides of outer shell 40 andthe space 47 between them may be filled with absorbing material. Channels 48 form passageways for the -gases to pass through the muflier. f l p Testsconducted with mu'lers of the type shown in Fig. l gave surprising results. Keeping constant the muffler lengthv and the thickness of the layer of gas-pressure absorbing and ,sound absorbing material adjacent the interior of the outer shell it was found that varying the diameter of the muiiier within reasonable limits produced no appreciable change in ultimate quantity of sound, measured y means of an acoustimeter, which emerged from the mutller when the latter Was connected in the ordinary manner in the exhaust system of aninternal combustion engine. .I-Iow'ever, when the unmuiiied exhaust noises were acoustically separated from the gases and conducted to the muiiier through an intake pipe similar to that used' 1n the prior tests, it was found that the quantity of sound emerging from the muler varied with the diameter of the muiier. From such datait appears that a mufiler of small diameter, while it does not permit'y much expansion and does not act as eiliciently upon lthe gas pulses, acts more efiiciently upon the noise created therein with ultimate muiiling equal to or better than that of the larger device which allows the as to expand more and thus acts more eiciently upon it but acts less eiiiciently upon the noises created therein. The muffler absorption for any one absorber may be increased by increasing the thickness of the absorber Within limits, or by increasing the outside diameter of the muiler within limits, or by increasing the length of the absorber'without changing its thickness, or by decreasing the gas duct diameter within limits.

While I have illustrated and describedbut a veryfew simple forms of my invention, it is understood that I may employ a great many forms since I believe that. I have inmuler with' gas-pressure absorbing and sound absorbing material adjacent the path of the exhaust gases and do not .wish to be confined to structural details. For instance, the inner shells may be of Wire screen, or other suitable foraminous material, or' may be entirely omitted as hereinbefore eX- plained, and the general shape f the device need not be cylindrical. Its cross section may be circular, rectangular, elliptical or of any other form and the manner of assembling the parts may vary considerably from that described hereinbefore. The rectan lar section is more effective than the circ ar section because of the greater area of absorber exposed to the gases in proportion to the cross-sectional area of the duct. My muffler consists essentially of a duct lined with gaspressure absorbing and sound absorbing construction Which may be a duct lined with absorbing material without a foraminous lining. It may be a fibrous gas-pressure absorbing and sound-absorbing lining combined with a foraminous or perforated section of engine exhaust pipe or any duct conducting noise-producing gases. For nonpulsating noisy gases such as are encountered in intakes of 4air compressors, the cutting down of a factor.

In any form of device I may choose to construct the cross-sectional area of the outlet should be large enough not to build up any appreciable back pressure. Many present muiiiers assist the silencing action by constricting greatly the cross-sectional area of the as outlet but this cannot be done without uilding up back pressure.

Although published data available relative to mechanics of noise production by internal combustion engine exhausts are meagre, I believe the hypotheses and explanations hereinbefore advanced to explain the manner in which my device accomplishes the muling of exhaust or ex losion noises are true. Whatever the theorles and explanations may be, the fact remains that, although a substantially straight and unobstructed path is offered the exhaust gases and noises they do not traverse that path unchanged but undergo a change therein which causes them to emerge almost, if not quite, noiseless.

Muillers which are considered satisfactory and used by automobile manufacturers, show by acoustimeter measurement, a total muling eciency of at least 55 percent when -the internal combustion (engines, for 'which the muiiiers were designed, are run at full load. A muiiler for an automobile engine which has a muflling eilicienc at full oad of percent is exceptional. lthough the gas pressure peaks is not the muling efficiency of satisfactory mulers for automobiles is only 55 percent at full load, it must be recognized', that in cities Where this efficiency should be high, automobile engines practically are never run at full load. Under light loads, characteristic of city driving, the efiiciency may be well over 95 percent. Even on country roads Where -high speeds may be attained, automobile engines are seldom subjected to full load. On the other hand, motor boat engines and especially outboard motors, are very often run at full load, so that unless the muffler efficiency is high, that is, above 95 percent, the exhaust noise is excessive. This noise is heard for a long distance over the Water and therefore is objectionable. The ordinary outboard' motor mufller, although seemingly ineflicient, has an efliciency of from 7 5 to 90 percent at the sacrifice of some power due to back pressure. Stationary internal combustion engines and others Whichare run so that they are Well loaded, usually must 'be Vequipped With high eiiiciency mufflers.

The efficiency of the muffler is determined by measuring the noise generated by the gases escaping into the atmosphere With and without muliing. The percent efficiency is the ratio of the decrease in measured sound to that measured Without mufiling. The muffler efliciency should be determined When the muffler is used with the engine for which it is designed. This is essential since the size, shape, amount of absorbing material, and other variables of the muffler are determined for each of the various sizes and types of engines.

Using .my invention, as hereinbefore described, it is ossible to construct mufilers of varying e ciencies, from 55 percent to Well over 95 percent by varying the quantity and type of absorbing material and by varying the construction as hereinbefore described. Such mufllers may be constructed so as to cause practically no increase in the back pressure on the engine at full load aside from that due to the surface friction of -the duct Walls. Furthermore, such niuiiiers occupy a small space, are light in weighttand may be built at a considerable saving in cost over the present-day muffler.

I claim: l

l. A muflier for the noise producing exhaust of an internal combustion engine comprising the combination With a substantially imperforate casing, of a substantially straight duct in said casing forming a substantially straight passage for said gas through said casing, said duct having a foraininous Wall and a contiguous backing of porous gas-pressure absorbing and sound absorbing material of sufficient thickness and porosity to cause gas pressure peaks in said gas to be suppressed primarily in the interstices of said absorbing material; said material occupying from 1 to 2() per cent of the volume of the space into Which it is packed, said absorbing material not causing any material increase in the resistance to the iow of said gas through said duct.

2. A muffler for the noise producing exhaust of an internal combustion engine coni prising the combination with a substantially imperforate casing, of a substantially straight duct in said casing forming a substantially straight passage `for said gas through said casing, said duct having a foraminous Wall and a contiguous backing of gas-pressure absorbing and sound absorbing material, said material Weighing from 4 to 100 pounds Aper cubic foot, said absorbing material not causing any material in crease in the resistance to the flow of said gas through said duct.

3. A mufiler for the noise producing eX- haust of an internal combustion engine comprising the combination With a substantially imperforate casing, of a substantially straight duct in said casing forming a substantially straight passage Jiior said gas through said casing, said duct having a perforated Wall and a contiguous backing of fibrous gas-pressure absorbing and sound absorbing material, said fibrous material occupying from l to 5 per cent of the volume of the space into which it is packed, said fibrous material not causing any material increase in the resistance of the flow of said gas throufrh said duct.

4.` mufiler for the noise producing exhaust of an internal combustion engine coniprising the combination With a substantial ly imperforate casing, of a substantially straight duct in saidcasing forming a substantially straight passage for said gas through said casing, said duct having a foraminous Wall and a contiguous backing of gas-pressure absorbing and sound absorbing material, said material Weighing -from 9 to 36 pounds per cubic foot, said absorbing material not causing any material increase in the resistance to the flow of said gas throufrh said duct.

5. n exhaust muffler for internal c0m bustion engines comprising a 'plurality of spaced, open-ended, co-axial, metal shells of equal length, the inner shell being perforated and the outer shell being substantially imperforate, metallic end closures between said shells, gas-pressure absorbing and sound absorbing material occupying from 1 to 20 per cent of the space between said shells, said material being uniformly distributed through said space and not causing any material increase in the resistance to the flow of gas through said inner shell.

, 6. A mufiier for the noise producing exhaust of an internal combustion engine comprisinfr the combination With a substantially imperiaorate casing, of a substantially straight duct in said casing forming a sublOl? stantially straight passage' for said gas through said casing, said duct having a Wall of gas-pressure absorbing and sound'absorbing/unaterial, said absorbing material being arranged to present an absorbing surface to gas flowing through said duct and consisting of porous packed material in layers of at least -1/4 inch thick and of sufficient porosity to cause gas pressure peaks to be suppressedy primarily by absorption in the interstices of said absorbing material, said material Weighing from 4 to 100 pounds per cubic foot and not causing any material increase in the resistance to the flow of said gas through said duct.

7. A mufller for exhaust gases of internal combustion engines having a mullling efficiency comparable with that of mufflers now the volume of the space between said duct i and said casing.

8. A mufller for exhaust gases of internal combustion engines having a muflling efliciency comparable with that of mullers now in use on commercial automobiles and capable of producing a marked decrease in back pressure when used in place of such mulllers, said mufiler comprising a substantially imperforate casing, a duct arranged in saidI casing and having a foraminous Wall, and a contiguous backing ,of gas-pressure absorbing and sound absorbing material between said foraminous Wall and said casing, said material Weighing from 4 to 100 pounds per cubic foot. 9. In ra mufiler for a noise producing gas the combination with a substantially imperforate'casing, of a substantially straight duet in said casing and forming a substantially straight passage for said gas through said casing, said duct having a Wall of gaspressure absorbing and sound absorbing material, said material occupying from `1 to 20 per cent of the volume of the space in which it is packed, said material not offering any material increase in the resistance to the flow of said gas through said duct.

10. In a muliler for a noise producing gas, the combination with a substantially imperorate casing, of a substantially straight duct in said casing and forming a substantially straight passage for said gas through said casing, said duct having a Wall of gaspressure absorbing and sound absorbing material, said material Weighing from 4 to 100 pounds per cubic foot, said material not causing any material increase in the resistance to the flow of said gas through said duct.

1.1A muiiler for the noise producing eX- haust of an internal combustion engine comprisingr the combination with a substantially imperforate casing, of a Asubstantially strai ht duct in said casing forming a substantially straight passage for said gas through said casing, said duct having a Wall of porous packed mineral fibers .Weighing from 4 to 100 pounds per cubic foot, said fibers not causing any material increase in the resistance to the flow of said gas through said duct.

12. A muier for noise producing gas comprising the combination With a substantially imperforate casing, of a foraminous duct extending through the casing and forming'a substantially straight passage therethrough for said gas, said duct having a contiguous hacking of porous packed metallic fibers, the individual fibers occupying from 1 to 20 per cent of the space occupied by the packed\ fibers whereby gas'pressure peaks `will be suppressed primarily by absorption in the interstices o said fibers, said fibers not causing any material increase in the resistance to the flow of said gas through said duct.

13. A mufller for noise producing gas comprisin the combination with a substantially imper orate casing, of a foraminous duct extending through the casing and forming a substantially straight passage therethrough for said gas, said duct. having a contiguous backing of porous packed metallic fibers Weighing from 4 to 100 pounds per cubic foot, said fibers not causing any material increase in the resistance to the flow of said gas throughJ said duct.

14. A muil/ler for ynoise producing gas comprising the combination With a substantially imperforate casing, ,of .a duct extending through the casing and forming a substantially straight passage therethrough for said gas, said duct comprising' Ametallic Wool in a layer of appreciable thickness and weighing from 4 to 100 pounds per cubic foot Iwhereby lgas pressure peaks will be sup-- pressed primarily by absorption in the in-'1 terstices of said metallic Wool.

15. A muler for noise producing gas comprising the combination with a substantially imperforate Casing, of a duct extending through the casing and forming a substantially straight passage therethrough for said gas, said duct having a foraminous Wall with an adjacent backing of porous packed steel wool, the individual fibers of said steel Wool occupying from 1 to 2O per cent of the spacel occupied by said steel Wool whereby gas pressure peaks in said gas Will be suppressed primarily by absorption in the interstices of said steelwool.

y 16. A mulller for noise producing gas comprising the combination with a substantially ilnperfor-ate casing, of a duct extending'to be suppressed primarily by abs orpt1on 1n through thel casing and forming a substanthe lnterstices thereofz and weighing from l Jtially straight passage therethrough for said to 100 pounds per cublc foot. A gas, said duct having a foraminous wall and In testimony whereof I aflix my signature. a backing of porous packed steel wool ar- FREDERICK H- SCHNELL 70 ranged adjacent said foralnino'us wall, the individual fibers of said steel wool occupying from 2 to 5 per cent of the space adjacent said foraminous wall whereby gas pressure 10 peaks will be suppressed primarily by ab- 75 sorption in the interstices of said steel Wool. 17. A muler for noise producing exhaust gas of an internal combustion engine comprising the combination with a substantially imperforate casing, of a metal duct extend- 80 'ing through the casing 'and forming a str-aight passage therethrough for said gas, said metal duct being provided with a perforated wall and having acontiguous backing of porous packed steel wool, said porous 85 packed steel wool being packed to Weigh from 4l to 100 pounds per cubic foot.

18. An exhaust muiller for noise producing exhaust gas of an internall combustion engine comprising a plurality of spaced co- 90 axial metal shells of equal length with metallic end closures between said shells, said end closures being pro-vided with central 'openings in alignment with said inner shell, 80 said inner shell being perforated and said 95 outer shell being substantially imperforate, 1 to 20 per cent of the space between said shells being occupied with the individual libers of porous packed steel Wool whereby gas pressure peaks of gases passing through 100 said inner shell Will be suppressed primarily by absorption in the interstices of said porous packed steel wool. 4 v.

19. A muilier for the noise producing ex- 40 haust gas of an internal combustion engine 105 comprising the combination with a substantially imperforate casing of a metal duct extending through the casing and forming a substantially straight passage therethrough for said gas, said duct being provided with 110 a perforated wall having a contiguous backing formed of a porous mattress of steel wool of suiicient thickness and porosity to` cause gas pressure peaks to be' suppressed primarily by absorption in the interstices 115 thereof, and Weighing from 4 to 100 pounds per cubic foot.

20. A muiler for the noise producing exhaust gas of an internal combustionengine comprising the combination with a substan- 120 tially imperforate casing of a metal duct extending through the casing and forming a substantially straight passage therethrough for said gas, the space between the duct and the casing forming an expansion chamber, 125

and a mattress of porous packed metallic fibers arranged in said expansion chamber and exposed to gas passing through said duct7 said mattress being of suflicient thickness and porosity to cause gas pressure peaks 130 

